Spider- A Layperson’s Insect

August 5, 2020 8:50pm IST

   'Is a spider an insect?' Most of us, if asked, will answer yes. But is the answer correct?

Let us try to find out. The word insect stems from the Latin word insectum, which means cut or divided into segments. Furthermore, the Latin word was derived from the Greek word entomon. Thus, scientists that study insects are called entomologists.

An insect's body is divided into three parts: the head, the thorax, and the abdomen, whereas a spider's, in two: the cephalothorax (combined head and thorax) and the abdomen. Insects have three pairs of legs, whereas spiders have four. Thus, it is clear that there are differences in their major body parts.

Now let us look into their minor body parts. Everyone knows that insects have two antennae, but have you ever seen a spider with antennae? Well, spiders have pedipalps. They may look like small legs, but their work is more like sensing objects. Moreover, insects use their mandibles to consume food. As for spiders, they use their chelicerae to consume their food. It may sound like both are performing the same function then why they have different names. Well, there is a difference in shape between the two; chelicerae are pincer-like while mandibles are jaw-like. Spiders lack mouthparts for chewing, unlike insects that have a pair of mouthparts for chewing and cutting. Due to this, spiders need to predigest food and nutrients in liquid form. They do this by entrapping their prey in the web and then injecting venom and covering the prey with the web. With time, the venom or secretions injected by the spider digest the prey's body. And thus, the spider gets the required liquid meal. Also, insects have a pair of compound eyes, but in the case of spiders, they have four pairs of simple eyes.

Well, now we know that spiders and insects have differences in their major as well as minor body parts.

Both insects and spiders have jointed legs and other similarities, which makes them fall under the same phylum of Arthropoda (Greek Arthon=joint; podos=foot). But due to their varying differences, they belong to different classes. Insects belong to the class Insecta while spiders are placed in the class Arachnida. So, spiders and insects are arthropods, but spiders are not insects.